1. Field of the Invention
The present invention relates to a method of preventing the formation of ring defects during the etching of a metalization layer after the formation of an Anti-Reflective Coating ("ARC") layer over the metal layer in preparation for patterning and etching the metal layer to form the desired metalization.
2. Description of Related Art
It is well known in the art to form an ARC layer over a layer of metal in preparation to patterning prior to etching the metalization pattern from the metal layer. The ARC layer may be of a very different coefficient of thermal expansion (CTE) than the metal layer, and is deposited at an elevated temperature. As a result the metal shrinks more than the ARC as the device cools. The ARC layer being relatively much thinner than the metal layer and much more hard and brittle is caused to crack. This cracking allows the photoresist developer in the subsequent patterning steps to seep through the overlying ARC layer and chemically react with the underlying metal. This has been found to leave behind generally cylindrical or ring-like deposits of the metal or an alloy or compound of the metal in places where the patterning should have resulted in etching away the metal. As line/space dimensions become increasingly smaller, e.g., in the sub-micron range, these defects increasingly cause low yields in the manufacture of integrated circuits using such metalization processes. The rings/cylinders form defects close enough to respective adjacent metalization lines to form shorts.
E. G. Colgan, B. Greco and J. F. White have reported upon such a phenomenon in "FORMATION MECHANISM OF RING DEFECTS DURING METAL RIE", 1994 VMIC Conference, ISMIC (June, 1994), the disclosure of which is incorporated herein by reference. This paper deals with the deposition of a titanium nitride ARC layer over an aluminum metalization layer comprising a copper-aluminum alloy of 0.5 to 1% copper deposited on a titanium layer and covered by a titanium layer ("Ti/Al(Cu)/Ti/TiN" or "TACT"). The authors there reported that the rings resulted from holes formed in the TiN after it was deposited due to the brittle nature of TiN and the large difference in the Coefficient of Thermal Expansion ("CTE") between the two substances. For example the authors noted that Al had a CTE of 24 ppm/.degree. C. and TiN had a CTE of 9.3 ppm/.degree. C. These holes formed by the splitting and cracking of the TiN layer allow the subsequent utilized photoresist developer to prematurely etch the Al in undesired places where the photoresist developer is exposed to the TiN layer in the process of patterning the metalization etching mask over the metal layer. Subsequently, according to this paper, the etched aluminum deposit is oxidized during rinse and forms Aluminum Oxide, Al.sub.2 O.sub.3, in cylindrical or ring-like shapes in a pocket under such a hole formed in the TiN. This aluminum oxide formation then masks a further etching step, e.g., through Reactive Ion Etching ("RIE"), producing the rings of mostly aluminum in the fields where no metal should be. The authors did not suggest a solution.
Applicants have experienced very similar problems in using a TiN ARC over a layer of metal formed of aluminum copper alloy in metalizing integrated circuits having sub-micron dimensions. The TiN has been placed upon the metal layer as an ARC at typically a temperature of 350.degree. C. or more. Because of the relatively large grain size of the Al-Cu alloy and the large difference in the CTE between the Al-Cu alloy and the TiN, the ring defects referenced in the above-noted paper are occurring in Applicants' process. Aluminum is first deposited on the wafer at 400.degree. C. in the I.C. manufacturing process. The wafer then is moved to the TiN deposition chamber, and the TiN is deposited at a starting temperature of above 350.degree. C. Depending upon the deposition thickness of the TiN, the final temperature in the chamber can be above 500.degree. C. Subsequently, as the stack of metal and ARC film cools, the aluminum shrinks much more than the ARC and the resultant grain collapse causes the harder and more brittle ARC to crack and split.